Abstract:Large-area electronics based on organic materials promise low-cost fabrication, lightweight construction, mechanical flexibility and durability. To truly realize the low-cost aspects of organic electronics, however, conventional high-vacuum deposition technologies – costly both in terms of instrumentation as well as operation – will have to be replaced by solution processing methodologies, like inkjet printing or spin casting. This need has in turn driven the development of solution-processable organic semiconductors, and even solution-processable organic conductors.

Given that organic semiconductors transport charge intermolecularly through pi-orbital overlap, the morphology that organic semiconductor thin films adopt critically affects the performance of organic thin-film transistors (TFTs) that comprise them. This talk highlights our efforts to control the crystallization of small-molecule organic semiconductors using post-deposition processing methods. Thin films of solution processable triethylsilylethynyl anthradithiophene (TES ADT) exhibit limited crystallinity as deposited. Exposing them to solvent vapors induces the formation of large-scale, two-dimensional spherulites; TFTs comprising these thin films display a hole mobility that is more than two orders of magnitude higher than those with as-cast thin films. Importantly, by patterning the substrate to have regions with different surface energies, we have been able to control the in-plane direction with which crystallization takes place. We have thus been able to engineer low- and high-angle grain boundaries to ascertain their contributions to charge transport. Using solvent-vapor annealing and other post-deposition methods, we have also been able to induce the controlled crystallization of contorted hexabenzocoronene (HBC) thin films. These films exhibit limited long-range order as deposited; depending on the details of the post-deposition conditions, we are able to tune the out-of-plane orientation of HBC without any surface treatment prior to deposition. Not surprisingly, the mobility of TFTs comprising these films is directly correlated with the extent of out-of-plane pi-stacking quantified in the active channels.

Biography:Lynn Loo is the Theodora D. ’78 & William H. Walton III ’74 Professor in Engineering at Princeton University. In the Chemical & Biological Engineering Department, her research emphasizes the structure development of complex materials for low-cost, lightweight and scalable plastic circuits and solar cells. With her recent stint at NewWorld Capital Group, a private equity firm that focuses on investments in environmental opportunities, her research has expanded to include macro-energy-systems analysis and carbon balance for processes that generate liquid fuels. As the Associate Director of External Partnerships at the Andlinger Center for Energy and the Environment, Professor Loo leads the Princeton Effiliates Partnership that promotes teacher-student-practitioner interactions and fosters collaboration between the private sector and faculty on campus. She received her PhD from Princeton University in 2001. She is a fellow of the American Physical Society and has been recognized as a Top 100 Young Innovator by MIT’s Technology Review, with the Alan P. Colburn Award from the American Institute of Chemical Engineers, the John H. Dillon Medal of the American Physical Society and a Sloan Fellowship, among other accolades.